I saw this, but it may not be a bug. It looks like if you trigger the artifact claiming from a border guard twice in a turn, the second one doesn't allow you to take a second artifact.
game id: 20039197
Turn 14, my opponent king's a border guard. The first one hits 2 actions and my opponent takes the horn. The second one also hits 2 actions, and my opponent takes nothing. Perhaps they choose to take nothing, but I doubt it (in chat we both said it was our first time playing with border guard).